SOFC type fuel cell systems (solid oxide fuel cell) are known, in which a material component is nickel. Such fuel cell systems can be sensitive to oxidation, resulting in nickel oxide. The oxidation of nickel can occur immediately when the surrounding gas mixture is not exclusively reductive (i.e., if it contains oxygen molecules available for an oxidation reaction). If nickel oxide inadvertently forms in a sufficient amount, the morphology of an anode electrode can change irreversibly. The electrochemical activity of the anode falls considerably, leading, in an exemplary worst case scenario, to a termination of the entire fuel cell operation.
Therefore, specifications of, for example, SOFC systems, in situations other than a normal running condition, include actions be taken in order to prevent such oxidation. An exemplary measure is to supply the anode side with a safety gas which contains reducing components capable of protecting the anode electrodes of a fuel cell from oxidation. In practice, the safety gas is used for bonding all of the free oxygen slipping or striving from the cathode side to the anode side by burning via the safety gas catalytically with the electrode. The reducing gas atmosphere established by a safety gas is used for the anode electrodes of a fuel cell in conditions with no actual fuel being supplied into the fuel cells. Exemplary conditions like that include a start-up and shutdown of the apparatus. The reducing component employed in a safety gas is, for example, hydrogen which uses a catalyst in order to react with oxygen and burn it away.
For reasons of safety, however, the concentration of hydrogen is diluted to a suitable level by an appropriate inert gas such as nitrogen. When the employed safety gas is a hydrogen-containing gas mixture, it would be, just from the aspect of safety, the more beneficial the lower the hydrogen concentration. Namely, the concentration of hydrogen gas is diluted to a level sufficiently low for staying at each temperature below the concentration matching the ignition point of hydrogen gas.
However, the dilute concentration of hydrogen involves a high-volume total flow because the amount of an inert gas, for example nitrogen, used for diluting the hydrogen, respectively increases. On the other hand, in order to limit the total volume of a safety gas, it would be the more beneficial the higher the hydrogen concentration. The use of a higher concentration of hydrogen would enable lessening the demand for nitrogen and further the total amount of safety gas.
In addition, when using such a safety gas in a known manner, the process operating window can become limited to an unnecessarily small size. The concentration of a safety gas should be controlled in such a way that the mixture flowing out of a possible leakage—fuel cells typically leak a certain amount of gases to their vicinity—shall retain its properties below the values matching the auto-ignition point—primarily below a LEL (Lower Explosive Limit), i.e., a lower auto-ignition point. For example, in the case of a hydrogen-nitrogen mixture at room temperature, this represents a hydrogen concentration of about 6%. As temperature rises, this threshold concentration becomes gradually even lower. Thus, the hydrogen concentration has quite strict limits imposed thereupon. Even moderately minor variations for example in hydrogen concentrations bring the parameters of a gas mixture too close to values corresponding to what is in excess of the above-mentioned ignition point. Thus, when using a hydrogen-containing safety gas, the operating parameters of a process, and for example the hydrogen concentration, should be subjected to a precise monitoring regime. This is particularly relevant at higher temperatures. Regarding the ignition of a safety gas, another exemplary aspect is the surrounding ambient temperature such as a space surrounding the fuel cell, into which space the safety gas is possibly able to leak.
After flowing once through a fuel cell, the spent safety gas is expelled from the fuel cell system. The expelled gas, along with the inherent outflow of fuel cells, can be conducted further through an afterburner in which the reductive gases are burned away and the heat is possibly recovered.
Accordingly, because of the operating principle applied in currently available systems, there is a high demand for the large total volume of safety gas. Thus, the use of a safety gas incurs major costs just in the form of raw materials. The high volumetric flow of a safety gas also means that the storage facilities therefor involve substantial space, which further results in additional costs and possible operational restrictions. This pressurized, most often hydrogen-containing safety gas is, for example, stored in compressed gas cylinders. Thus, aboard ships, for example, the amount of safety gas is limited both by a large space for storage and by safety issues.